Should I use a bit mask when truncating uint64_t to uint8_t[i]?

Question

If I have a large int, say a uint64_t, and an array of uint8_t, e.g.:

uint64_t large = 12345678901234567890;
uint8_t small[5];

and I want to copy the 8 least significant bits of the uint64_t into an element of the array of uint8_t, is it safe to just use:

small[3] = large;

or should I use a bit-mask:

small[3] = large & 255;

i.e. Is there any situation where the rest of the large int may somehow overflow into the other elements of the array?

Answer 1

It will most certainly not cause data to be processed incorrectly. However, some compilers may generate a warning message.

There are two options to avoid these.

You can cast your variable:

(uint8_t)large

Or you can disable the warning:

#pragma warning(disable:4503)

I would suggest casting the variable, because hiding compiler warnings will potentially keep you from spotting actual problems and is therefore not best practice.